Specifically, they argue that the Nasdaq's plan to compensate investors is insufficient.

From the letter:

Market participants suffered hundreds of millions of dollars of losses as a result of Nasdaq's profit-driven conduct prior to and during the Facebook IPO, not as a result of protected regulatory activity by Nasdaq, or routine system failures, ... Nasdaq was grossly negligent in its handling of the Facebook I.P.O., and as such, Citi should be entitled to recover all of its losses attributable to Nasdaq's gross negligence, not just a very small fraction as is currently the case.